This invention relates to a microwave oven used for cooking and, particularly, to a waveguide filter used in a microwave oven for eliminating harmonic noises created by the magnetron and preventing microwave radiation outside of the heating chamber.
The frequency (fundamental wave) assigned to the operation of microwave ovens is 2.45 GHz.+-.50 MHz. A magnetron used in a microwave oven produces frequency components (noises) other than the fundamental component although their levels are extremely low. Leakage of such noises from a microwave oven is seriously influential upon other electronic appliances, and therefore various measures are taken in designing a microwave oven. Conventional microwave ovens are provided with a filter within a waveguide which conducts the microwave from the magnetron to the heating chamber so as to eliminate noises, as described, for example, in Japanese patent publication Nos. 59-16713 and 59-16714. Although such a filter arrangement is effective against noises relatively close to the fundamental wave, it is not intended to eliminate higher harmonic components.
Recently, broadcasting satellites have been launched, and television signals are transmitted directly from each satellite to the home audience. The frequency band assigned to broadcasting satellites is 11.7-12.7 GHz, and it coincides with the fifth harmonic, i.e., 12.0-12.5 GHz, of microwave ovens. Therefore, a leakage of the fifth harmonic from a microwave oven can adversely affect television sets which are tuned to satellite broadcasting stations.
In order to eliminate the spurious emission from microwave ovens, there has been proposed a broadband filter as described in U.S. Pat. No. 3,758,737. This technique, however, does not cover high order modes and is relatively expensive.